It's a group of homerooms.
Basically u r assigned a HR and a cluster. And there are four HR's per cluster and 4 teachers ( for math, science, social studies, ELA) u can have classes with anybody in ur cluster but u can't have classes with kids who aren't in ur cluster
In sixth grade, u go from class to class with ur HR. In seventh grade there could be some kids in ur HR who aren't in any of ur other classes